The invention is related to a digital telecommunication system. More specifically, it is related to a digital telephone apparatus suitable for subscriber stations and attendant consoles. Still more particularly, it is related to a transceiver forming an interface between the digital telephone apparatus and a telephone speech transmission line for duplex communication.
Such a transceiver receives a serial data stream from telephone apparatus and performs pulse shaping and converting operations on such a data stream prior to transmitting it onto the speech transmission line. In the opposite transmitting direction a serial data stream is received from the transmission line and is then processed to obtain a non-distorted pulse train bearing binary coded data in a code compatible to the requirements of the telephone apparatus.
The digital data are transmitted at a fixed data rate which is inherent for the telecommunication system and are arranged in frames such that binary coded information and control information of the communication system like a synchronization code and a signalling code can be definitely separated by allocating items of data to predetermined time slots.
Conventional transmitting procedures of digital data are known which do not require transmitting of a separate clock which has to be synchronized with the system clock but make use of possibilities for recovering the system clock pulses from the transmitted data. But digital telephone apparatus of the state of the art cannot receive and transmit digital data in different channels and in one frame format by means of a signal transmitting receiving device designed as an interfacing unit between the digital telephone apparatus and the transmission line for duplex communication.
A variety of encoding techniques for transmitting digital data is known; one of these techniques is the so-called "Alternate Mark Inverted" (AMI) encoding which has been used for digital trunk circuits in pulse code modulated (PCM) communication systems. A main characteristic of AMI encoding is that the required bandwidth of the transmitting medium is only one-half the bandwidth of the digital data to be sent. For example, in a PCM 30 system--wherein one pulse frame consists of 32 data bytes the conventional data rate is 2.048 MHz based upon the usual sampling rate of 8,000 samples. If AMI encoding is used, the necessary bandwidth is only 1.024 MHz. Thereby, longer cable distances between repeaters can be achieved and cross-talk between twisted cable pairs of the transmission line which escalates with increasing frequency can be minimized.
It is, therefore an object of the present invention to provide an integrated transmitting/receiving unit, which is called a transceiver of low outlay and high reliability which is designed especially for use with peripheral units of such systems, such as subscriber sets and attendant consoles.
Another object of the present invention is to provide such a transceiver which is suitable for use with digital telephone apparatus which may be connected to the control switch by transmission lines comprising different loop lengths in a range of up to 4,000 feet without the need of individual adjustments according to the varying loop length.
Still another object of the present invention is to provide such a transceiver which dissipates little watt power, which meets official standards and requirements, also for heat radiation, and which is designed in a space-saving manner.
Still another object of the present invention is to provide a transceiver which is designed for recovering system clock pulses from an incoming data stream. Thus clock pulses are obtainable for use as internal clock pulses in the transceiver for synchronizing the serial data received from and to be transmitted to the transmission line and for use as internal clock pulses within the telephone apparatus as well.